This invention relates to a light quantity control apparatus, more particularly apparatus for controlling the quantity of light to be transmitted in an optical communication system.
With recent improvement of the performances of optical fibers and semiconductor laser devices, optical communication systems have progressively been used in the field of practical applications.
In such an optical communication system it is not only necessary to transmit light without attenuation but also to modulate the quantity of light, or to branch the light into two portions or to switch light among different light paths.
According to a prior art light quantity control apparatus one or two birefringence elements are combined with a light modulator to control the ratio of output light to the incident light to the control apparatus. A combination of one birefringence element and one light modulator is disclosed in a J. J. Nelson's paper entitled "Digital Light Deflection", "The Bell System Technical Journal", May, 1964 Vol. XLIII No. 3, pages 821-845 and an example of a light control apparatus including two spaced apart birefringence elements and light modulator interposed therebetween is disclosed in a Takao Matsumoto's paper entitled "Investigation of a Minute Light Circuit Utilizing Birefringence Characteristic" presented before a meeting of the Society of Japanese Electronic communication and published on Sept. 11, 1978.
In the former, however, the incident light must be linear polarized light, whereas in the latter, although it is possible to effect light control, such as branching, without relying upon polarized incident light, the output of the second birefringence element comprises first incident light consisting of either light subjected to a polarization conversion by the light modulator or light not subjected to the polarization conversion, and second and third output light waves independently produced in response to two input light waves and different from the first output light.
As above described, usually one light wave is separated into two components applied along different light paths, or light is selectively applied to different light wave paths, or the quantity of one light is controlled. For this reason, with the prior art control device disclosed in the latter paper, the first output light wave is combined with either one of the second and third output light waves so that either one of the second and third output light waves should be discarded. The same is true when either one of the second and third light waves is selected to control the quantity of light.